1. Field of the Invention
This invention pertains to an n-line superheterodyne transmission and reception device in the communications field, especially the telecommunications field.
2. Background of the Invention
N-line (i.e., multiple line) superheterodyne receivers with n-line frequency conversion, such as are used with radio telephone devices, are well known in the prior art.
For example, a wireless set with a single superheterodyne receiver is described in European Patent Office application EP-B1-0 089 649, wherein the high frequency oscillation of the transmission oscillator is positioned between the receiving frequencies of two adjacent radio channels and is fed to a mixer. In view of the double use of the carrier oscillation which is generated in the transmission branch, not only can an oscillator be omitted in the receiving branch, but also interference due to a plurality of oscillators can be eliminated. However, such a circuit concept is disadvantageous as the intermediate frequency signal includes modulation signal components generated by the transmitter in addition to the modulation of the received signal.
In order to eliminate interference, particularly cross-talk, from a received voice signal on a wireless set through loudspeakers, the receiver must eliminate or suppress modulation signal components which are coming from the transmitter.
In practice, two different methods have been shown to be useful. The first method include the subtracting of the signal fed to the transmission modulated by the output signal of the demodulator in the low frequency range. The second method includes modulation by a second mixer with a counter phase signal for the modulation of the transmission signal. Both of these methods are known and are disclosed in German Patent DE-AS 1,002,805.
The wireless set disclosed in the aforementioned DE-AS 1,002,085 is operable as a wireless relay station or could communicate via a telephone line. Therefore, the high frequency carrier signal of the transmission branch which is modulated with its own signal is also superpositioned with the received signal for generating an intermediate frequency in addition to the generation of the transmission signal. This superimposed signal which is taken from the transmission branch includes the modulation corresponding to the transmitted signal. For compensation of the modulated component caused by the modulation signal, two embodiments of a compensation apparatus are disclosed in the aforementioned DE-AS 1,002,805 which are provided between the transmission and reception branch and which each include an amplifier and a phase shifting member.
The first method and apparatus includes a pure frequency modulation active compensation, while the second method and apparatus includes an additional modulation operation for compensation.
The first method is advantageous in that the individual electronic components must be design for use only in the low frequency range. Therefore, less expensive components may be used. However, this method is disadvantageous in that the running times caused by the ZF-filters must be compensated by running time components. This again causes a relatively high overall cost for the circuitry and further requires adjustment of the compensation apparatus.
The second method is advantageous because the modulation and the compensating signals pass through the second intermediate frequency filter simultaneously. In comparison with the first method, the compensation of the remaining running time differences are substantially simplified. However, it is disadvantageous because the frequency of the second receiving device must be very stable. Therefore, a high constant quartz oscillator is used as a second quartz oscillator. In order modulate such a quartz oscillator, a very expensive circuit is required. Moreover, the resulting circuit is disadvantageous in that the characteristic curve is non-linear.
Finally, a transmitting and a superheterodyne receiving device is disclosed in EP-B1-0 135 816, wherein the compensation device is provided with a modulator. The modulated output signal of the transmission oscillator, e.g., the transmission signal is modulated with the compensation signal (which, after passing through an amplifier and the subsequent phase shifting element, exits at its output) so that the superheterodyne receiving device is fed to a substantially remodulated signal. The frequency of this signal corresponds to the frequency of the transmission oscillator.